Nasser al-Mutairi arrived in Kuwait city overnight Sunday on a jet sent by the Kuwaiti government, head of the Kuwaiti Family Committee Khalid al-Odah said.
Mutairi was one of a dozen Kuwaiti men imprisoned at the US military prison in Cuba during the 2001 US war to oust al-Qaida and the Taliban from Afghanistan after the 11 September attacks.
Designated as "enemy combatants" by the Pentagon, most of about 550 non-US detainees at Guantanamo Bay base have been held without charge, legal representation or trial.
"This is one giant step in our efforts to obtain due process for the 11 prisoners left behind but it is only one step," Odah said in a statement.
He said none of the 12 were wanted for anything in their homeland and have no criminal records.
The statement said Mutairi, an Education Ministry employee, was also a missionary representing the apolitical Tableeghi sect of Islam when he travelled to Afghanistan in October 2000 to teach in the mosques and schools. There he was captured by bounty hunters and sold to US forces.
Rights groups say the US military
uses torture at Guantanamo Bay
"It's hard to put into words what he [Mutairi] and his family have been through these past three years," Odah said.
"Now we are praying that the Bush administration will either bring to trial or free the 11 other loved ones who are being held without their legal rights in Guantanamo."
Mutairi, 28, was accompanied by Kuwaiti police on the trip home and was whisked away by officers to be debriefed after shortly meeting with family members at the airport, Odah said.
"All the guys are fine and send you their regards," Odah quoted Mutairi as saying, referring to the remaining detainees.
"Everybody who's been there has been tortured"
Kuwaiti Family Committee head Khalid al-Odah
The committee headed by Odah has lobbied for the release of the Kuwaiti detainees at Guantanamo.
Odah's son, Fawzi, a 27-year-old religious studies teacher, was arrested in Pakistan near the Afghan border in December 2001 and is at the base.
Odah said the Kuwaitis held at Guantanamo, like captives from other nations, were subjected to physical and other abuse.
"Everybody who's been there has been tortured ... they denied their rights, they put them in very bad conditions," Odah said, adding: "They were even physically tortured."
Odah said the only contact the detainees had with their families were heavily-censored letters and through news carried by two official Kuwaiti delegations that visited them.
The International Committee of the Red Cross, which began visiting the Guantanamo detainees in early 2002, has accused the US military of using tactics "tantamount to torture" on prisoners at the US naval base.
The Pentagon has strongly denied the allegations of abuse there.